In the fabrication of semiconductor devices, particularly in the fabrication of sub-micron scale semiconductor devices, profiles obtained in etching processes are very important. A careful control of the surface etch processes is therefore necessary to ensure directional etching. In conducting an etching process, when an etch rate is considerable larger in one direction than in the other directions, the process is called anisotropic. A reactive ion etching (RIE) process assisted by plasma is frequently used in anisotropic etching of various material layers on top of a semiconductor substrate.
The plasma generated in a RIE process consists of neutrons, ions and electrons. The effect of each species on the etch results and their possible interactions with each other are not well understood. An etch process and the resulting etch products can be analyzed by mass spectrometry, while the chemical compositions on the surfaces can be detected by an in-situ or x-situ x-ray photoelectron spectrometer, Auger spectroscopy and secondary ion mass spectrometry. In plasma enhanced etching processes, the etch rate of a semiconductor material is frequently larger than the sum of the individual etch rates for ion sputtering and neutral etching due to a synergy in which chemical etching is enhanced by ion bombardment.
In an anisotropic etching process, it is clear that the ions striking the surface must themselves be anisotropic, i.e., the ions travel mainly in a direction perpendicular to the wafer surface. The ions are normally oriented and accelerated in a sheath. Proposed mechanisms for anisotropic plasma etching are that first, perpendicular ion bombardment creates a damaged surface that is more reactive toward neutral etchants, and second, ions help to desorb etch-inhibiting species such as etch products from the surface. In either one of the mechanisms, the ion path must be perpendicular to the wafer surface such that only the etch rate of the bottom surface is enhanced. Ideally, the ions should not bombard the sidewalls at all.
In a real plasma etching process, ion-neutral particle collisions from the plasma sheath result in a fraction of the ions bombarding the sidewalls. As a result, lateral etching of the sidewalls occurs to some extent. Theoretically, the number of ion-neutral collisions in the sheath is directly proportional to the sheath thickness and inversely proportional to the ion mean free path. Since the ion means free path is usually proportional to the chamber pressure, reducing the pressure results in reduced ion-neutral collisions and therefore enhances anisotropic etching. Since ions desorb etch-inhibiting species (such as etch products) from the etch surface, the formation of sidewall films (i.e., a passivation layer) during the etching process plays an important role in the development of the anisotropic etch profile. The passivation film protects the sidewalls from etching by non-perpendicular incoming ions.
In an anisotropic etching process, the etch directional control can be enhanced by a mechanism known as sidewall passivation. By adjusting the etchant gas composition and the reactor parameters, an etch-inhibiting film can be formed on the vertical sidewalls. The etch-inhibiting film (or passivation film) slows down or completely stopping lateral attack while the etching of horizontal surfaces (i.e., a bottom surface) proceeds. For instance, in an etch process for silicon, when O.sub.2 is added to a Cl.sub.2 plasma, an oxide film can be grown on the sidewalls that are not exposed to ion bombardment. Similarly, in a fluorocarbon plasma etching process, a greater elemental ratio of carbon to fluorine can be used to deposit involatile polymer films on the sidewalls thus forming a coating that blocks chemical attack. While polymer film may also deposit on the horizontal surfaces, it is readily removed by the ion bombardment and therefore allowing etching of such surfaces to continue.
The sidewall passivation effect is also observed when an insulating layer such as a photoresist layer deposited on top of a semiconductor substrate is bombarded by plasma ions. On top of the photoresist layer, charge built up occurs during the reactive ion etching process by the severe ion bombardment on the substrate surface. These stored charges can cause a distortion in the ion path bombarded toward the substrate surface. When positive charge accumulates on the wafer surface as a result of impinging ions and emitted secondary electrons, the photoresist surface may be charged up high enough to produce a current flowing through the photoresist layer causing its degradation or other permanent damages. The charge accumulation also causes a distortion in the path of the ion beam by stripping the space-charge compensating electrons from the ion beam. Such charge accumulation further distorts the ion beam path and causes them to collide with the sidewalls of the device.
In a plasma bombardment process conducted on a semiconductor structure covered by a photoresist layer, the plasma ions excited by the bias voltage etch away difficult-to-etch residue materials from the semiconductor substrate, while simultaneously etches a fraction of the photoresist material from the photoresist layer to perform sidewall passivation in order to enhance the anisotropic etching process. In the process where the photoresist material is etched away from the top of the substrate, two steps are normally involved. In the first step, the photoresist material is etched away and separated from the photoresist layer and deposited on the top portion of the sidewalls. The deposited photoresist material is then bombarded again by the plasma ions and sputters to the lower portion of the sidewalls. Since photo resist layer normally consists of a polymeric material and that when it decomposes in plasma, fragments of the polymeric material combine with some of the gas elements in the plasma to form a passivation material for depositing on the sidewalls.
In modern semiconductor devices where spacing between features are continuously being reduced, and when the photoresist layer is highly charged by the plasma, plasma ions that normally bombard in a perpendicular direction with the substrate surface no longer travel in a vertical manner. The paths of the plasma ions are distorted and instead of bombarding the bottom horizontal surface, some of the ions bombard the sidewalls of the etched feature (i.e., a metal line) on the substrate. The sidewall profile is attacked by the ions and as a consequence, the passivation layer coated on the sidewalls for protection purpose is first knocked off by the distorted ions. A chemical etch process by the neutral etchant then etches away the sidewall materials. The smaller the feature size, and thus the smaller the spacing between features on a substrate, the more severe is the plasma ion distortion problem and the sidewall etching problem.
An enlarged, perspective view taken from a SEM micrograph of several metal lines etched on a semiconductor substrate by a conventional etch method is shown in FIG. 1A. Metal lines 10 are formed on the surface of a semiconductor device 12 by a main etching and an over-etching step. In the main etching step, a metal stack of TiN/Al/Cu/TiN/Ti is etched by an etchant mixture of N.sub.2 /BCl.sub.3 /Cl.sub.2 which is superior in etching aluminum lines. The same etchant gas mixture used in etching AlCu and AlCuSi alloys. During the etching process, an excessive charge accumulation on the photoresist layer (not shown) that was deposited on top of the metal layer for the photolithography process distorts the plasma ion paths that was supposedly aimed perpendicularly toward the substrate surface. In ideal situations, only the bottom horizontal surface is to be etched by plasma ions that are not distorted. However, the charges carried by the photoresist layer diverts or distorts the plasma ions such that they bombard the passivation layer (not shown) coated on the sidewalls. After the passivation layer is etched away or severely damaged, etchant gas chemically attacks the metal layer such that voids or cavities 16 are formed. These voids or cavities cause quality problems such that the semiconductor device 12 may not function properly after the manufacturing process is completed.
The etching process for the semiconductor device shown in FIG. 1A is conducted under conditions illustrated in Table I.
TABLE 1 ______________________________________ Step P mt TCP W Bias V Cl.sub.2 BCl.sub.3 N.sub.2 Ar seconds ______________________________________ Main Etch 10 500 150 75 15 20 End Point Over Etch 10 300 150 40 30 10 60 Dechuck 10 100 0 100 10 ______________________________________
In the main etching process, chamber pressure of 10 m torr is maintained under a TCP power of 500 watts. The bias voltage utilized in the main etching process is approximately 150 volts. An etchant gas mixture for the main etch process consists of 75 sccm Cl.sub.2, 15 sccm BCl.sub.3 and 20 sccm N.sub.2. The etching process is stopped by an end-point mode. After the completion of the main etching process which stops at a metal/TiN interface, an over etching process carried out for approximately 60 seconds is conducted. In the over etching process, a chamber pressure of 10 m torr and a TCP of 300 watts are used. The bias voltage utilized is the same as that used in the main etching process, i.e., 150 volts. The etchant gas used contains less Cl.sub.2 and more BCl.sub.3 when compared to the etchant gas mixture used in the main etching process, i.e., 40 sccm Cl.sub.2, 30 sccm BCl.sub.3 and 10 sccm N.sub.2. The over etching process etches away the TiN layer at where it is not covered by the photoresist layer. The over etching process is stopped by a time mode at approximately 60 seconds. After the two-step etching process is completed, a dechuck process is carried out in order to eliminate the electrostatic charge on the wafer and to enable the removal of the wafer from the wafer chuck. During the dechuck process, a chamber pressure of 10 m torr is kept and a TCP power of 100 watts is used. No bias voltage is applied. An argon gas is flown into the chamber at a flow rate of approximately 100 sccm. The argon gas dechucks the wafer in approximately 10 seconds so that the wafer can be removed from the etch chamber.
It is therefore an object of the present invention to provide a method for etching metal lines with enhanced profile control that does not have the drawbacks or shortcomings of the conventional etching methods.
It is another object of the present invention to provide a method for etching metal lines with enhanced profile control by eliminating the charge built-up in the photoresist layer deposited on top of a semiconductor structure.
It is a further object of the present invention to provide a method for etching metal lines with enhanced profile control which includes a main etching and an over etching step by incorporating a neutralization step for discharging the charges carried by the photoresist layer.
It is another further object of the present invention to provide a method for etching metal lines with enhanced profile control wherein the etching process includes a main etching step which stops at a metal/TiN interface.
It is yet another object of the present invention to provide a method for etching metal lines with enhanced profile control wherein the etching process includes an over etching step for etching the metal lines through a TiN layer.
It is still another object of the present invention to provide a method for etching metal lines with enhanced profile control wherein the metal layer is deposited of a material of Al, AlCu or AlCuSi.
It is still another further object of the present invention to provide a method for etching metal lines with enhanced profile control wherein the metal lines consist of a metal stack of TiN/metal/TiN/Ti with the Ti layer next to the semiconductor substrate.
It is yet another further object of the present invention to provide a method for etching metal lines with enhanced profile control wherein a photoresist layer deposited on a metal layer generates a passivation material for coating the sidewalls of the metal line when bombarded by a gas plasma.
It is still another further object of the present invention to provide a method for etching metal lines with enhanced profile control by neutralizing charges build up in a photoresist layer by an inert gas plasma of argon or helium.